TM4SF19 and rheumatoid arthritis: Among the Toll-like receptor (TLR) family, TLR1-9 are expressed on osteoclast precursor cells.12 TLR4 is known to be involved in osteoclast formation, growth, differentiation, survival, and bone resorption.38,39 TLR4 deficiency suppresses bone loss by inhibiting RANKL expression.38–40 Blockade of the TLR-4/MyD88/NF-kB signaling pathway inhibited osteoclastogenesis.41–44 Therefore, our data suggest that increased expression of Tm4sf19 protein in synovial macrophages in rheumatoid arthritis amplifies TLR4-mediated inflammatory signals, thereby worsening rheumatoid arthritis.